1. Field of the Invention
The invention relates to a light source device for a liquid crystal projector in which a short arc metal halide lamp with bilateral sealed terminations is used for a light source.
2. Description of Related Art
In a short arc metal halide lamp with bilateral sealed terminations, at least one metal halide is encapsulated as the emission metal in an arc tube which is made of quartz glass and in which a pair of electrodes is located, at a distance of roughly a few millimeters from one another, together with mercury and a starting rare gas. Seal areas are joined in one piece to the two ends of the arc tube. In the respective seal area, a molybdenum foil is inserted, to the ends of which an electrode and one end of an outer lead pin are welded for purposes of power supply. Thus, the outer lead pin projects out of the respective seal area.
In a metal halide lamp of this type, as a result of the vaporization of the metal halide due to heating, a vapor pressure which is lower than in the case of using at least one metal element is obtained to a sufficient degree. In this case, the radiant efficiency is higher than in a high-pressure mercury lamp. In addition, by suitable selection of the metal to be encapsulated, outstanding color reproduction is obtained. Therefore, a metal halide lamp is often used for a light source device for a liquid crystal projector.
A light source device for a liquid crystal projector comprises, as is shown in FIGS. 8 and 9, a short arc metal halide lamp 10 with bilateral sealed terminations, as mentioned above, and a concave reflector 20 which has base opening 21 in the middle area. Metal halide lamp 10 is held securely in the state in which the axis of metal halide lamp 10 and the optical axis of concave reflector 20 roughly agree with one another, such that seal area 12 of metal halide lamp 10 is installed in base opening 21 by embedding using a filler material or by similar methods. The other seal area 13 of metal halide lamp 10, therefore, extends in the direction to front opening 22 of concave reflector 20. Outer lead pin 15, to which one end of conducting wiring 30 is connected, projects out of this seal area 13.
The conducting wiring 30 connected to projecting outer lead pin 15 is first withdrawn in the vertical/radial direction and then parallel to metal halide lamp 10, then passes through base opening 21 of concave reflector 20 and is drawn out to behind concave reflector 20, as is shown in FIG. 8. Alternatively, conducting wiring 30 is withdrawn from the end of seal area 13 in a large arc, and then passes through a through opening 24 which is formed on reflection surface 23 of concave reflector 20 is drawn out to behind concave reflector 20, as is shown in FIG. 9. This means that the conducting wiring 30 is withdrawn, in any case, from the end of seal area 13 of the metal halide lamp 10 and is in a state in which it is spaced very far from seal area 13 and from arc tube 11.
In a liquid crystal projector device, as is illustrated in FIG. 10, parallel light which has been reflected by concave reflector 20 passes through a condenser lens 51 and is incident on a liquid crystal cell 60, with an image which is displayed on a screen (not shown) by projection lens 52. Recently however, a liquid crystal projector device is being used more and more frequently in which an integrator lens 40 is provided between the concave reflector 20 of the light source device and the condenser lens 51 in order to make the distribution of illumination intensity as uniform as possible on the screen. Integrator lens 40 has a pair of lenses, i.e., an incidence side lens 41 and an exit side lens 42, in which several lens elements arranged flat. Furthermore, besides the above described components, if necessary, there are also parts such as a color filter and a polarization element and the like.
In an integrator optics system, the lens elements of the incidence side lens 41 are imaged by means of the exit side lens 42 on liquid crystal cell 60. For the light which emerges from this integrator lens 40, the distributions of the illumination intensity of the light which has emerged from several lens elements which form the integrator lens 40 are therefore added. This means that, in doing so, a distribution curve of illumination intensity is formed in which the mildly curved distribution curves of the illumination intensity of the exit light have been assembled by small phase differences from one another. Thus, the distribution curve of illumination intensity achieves a roughly flat shape. Therefore, the illumination intensity on the screen can be made roughly uniform.
The incidence side lens is also called a second light source, because the lens elements of the lens on the incidence side are imaged in this way by the exit side lens on the liquid crystal cell. Therefore, if there is a body in the vicinity of the lens on the incidence side as a second light source, its picture is imaged on the liquid crystal cell and therefore appears as a shadow on the screen, the shadow becoming more distinct, the smaller the distance between the body and the lens on the incidence side.
In a liquid crystal projector device, as a result of the requirement for reducing the size of the device, the design is also made such that the distance between the light source device and the lens on the incidence side of the integrator lens is a small as possible. The distance between the tip of the front opening of the concave reflector and the lens on the incidence side of the integrator lens is roughly 20 mm. Furthermore, the distance between the area in which the conducting wiring is removed and the lens on the incidence side is roughly 10 mm, since the conducting wiring connected to the outer lead pin proceeds from the end of the seal area, the outer lead pin projecting out of the seal area which extends in the direction to the front opening of the concave reflector as was described above. This means that this distance is extremely small. Therefore, the disadvantage arises that the shadow of this conducting wiring appears clearly on the screen.
Therefore, if the length of the seal area of the metal halide lamp is reduced, the distance between the conducting wiring and the lens on the incidence surface is greater, and the shadow on the screen becomes weak.
In a metal halide lamp in which the tube wall load is great and which reaches an extremely high temperature during luminous operation, when the length of the seal area decreases, however, the molybdenum foil inserted in the seal area is exposed to high temperature oxidation; this shortens the service life of the lamp. Attenuation of the shadow by reducing the length of the seal area is therefore limited.
Furthermore, for a metal halide lamp of the short arc type, a trigger electrode is ordinarily used as the outer auxiliary electrode to improve the starting characteristic. Between the arc tube of the metal halide lamp and the concave reflector there are, therefore, two wires, that is, the conducting wiring and the trigger electrode. However, here, the disadvantages arose that these two wires hinder the light incident from the metal halide lamp on the concave reflector and that the degree of utilization of the light decreases. Furthermore, there was the disadvantage that production of the trigger electrode is very costly.
Additionally, it is considered a disadvantage that, when using a liquid crystal projector as the light source device, the shadow of the conducting wiring is displayed on the screen because the conducting wiring which is withdrawn at a great distance from the arc tube is in a position which is nearer the integrator lens.